1. Field of the Invention
The present invention relates to a thin-film transistor, and particularly to a polysilicon carbon source/drain heterojunction thin-film transistor.
2. Description of the Prior Art
Amorphous silicon (a-Si) is used in the deposition of thin-film transistor (TFT) for liquid-crystal displays. Amorphous semiconductors do not have a crystalline lattice, and this specific disordered nature results in a large number of localized states with energy within the energy gap. The localized states in the upper and lower half of the gap behave like acceptors and donors respectively, which are used to implement a switching element for the liquid-crystal display (LCD). As the density and the quality of the pixels on the LCD increase along the advancement of the semiconductor industry, the amorphous silicon is no longer adequate due to its low mobility. High-mobility polysilicon is therefore a candidate for manufacturing TFTs on the liquid-crystal display.
FIG. 1A shows the cross-sectional view of a conventional TFT with a gate, a n.sup.+ polysilicon drain region and a n.sup.+ polysilicon drain region designated respectively as 10, 12 and 14. At least 5 volts is usually applied between the drain 14 and the source region 12 during its operation. When the TFT is in an off state with the gate-to-source voltage V.sub.GS less than the threshold voltage V.sub.th, the potential of the aforementioned drain-to-source voltage V.sub.DS almost crosses an depletion region near the drain region 14, thereby generating a large electrical field, and a large number of associated electron-hole pairs. FIG. 1B shows an energy band diagram when no V.sub.DS voltage bias is applied and the gate voltage V.sub.GS is less than V.sub.th, where the TFT behaves as a natural p-n junction, and where subscript c designates the conduction band, v the valence band, F the firmi level, and n, p the polarity of the material for the TFT.
As the gate voltage V.sub.GS is less than V.sub.th and the drain 14 -to- source 12 is biased in a normal operation, most part of V.sub.DS is dropped at the p-n depletion junction near the drain 14. The aforementioned generated holes on the left p side are apt to go over the barrier according to the quantum tunneling effect due to its narrow width 16 as shown in FIG. 1C, even an offset regions 18 (FIG. 1A) are provided.